Small DC motor

ABSTRACT

A small DC motor includes: a motor frame including a cylindrical portion, the cylindrical portion having a constant thickness and having a cross section in a shape that includes four sides and connecting portions, each of the connecting portions connecting adjacent two of the four sides and being located inward from a corresponding corner in a quadrangle including the four sides; field magnets; and an armature assembly, wherein the field magnets are provided so as to be spaced apart from each other, and the small DC motor includes an air gap between each of the four sides and a radially outermost surface of the armature assembly, the air gap being a minimum size needed to rotate the armature assembly.

This application is a continuation application of Ser. No. 12/320,254,filed Jan. 22, 2009, which is a continuation application of Ser. No.11/483,670, filed Jul. 11, 2006, now U.S. Pat. No. 7,498,706. Theforegoing applications are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a small DC (direct current) motor, and moreparticularly to a small DC motor which is small in size but which has alarge torque.

2. Background Art

In the field of small DC motors, there is an increasing tendency todemand motors small in size and light in weight. In DC motors which havecylindrical motor frames having circular cross sections, there is causeda limitation on a relationship between the shape of a field magnet andnecessary magnetization characteristics, and in addition, a separatemounting configuration becomes necessary.

In contrast, in DC motors which have motor frames having quadrangularcross sections, necessary magnetization characteristics or magneticproperties which appear as if they matched a sinusoidal magnetizationcurve can be generated by increasing the thickness of a field magnet ata corner portion of the motor frame, so that the motor frame can beformed into a shape which produces necessary magnetizationcharacteristics. In addition, since the cross section of the motor frameis quadrangular, any of sides of the motor frame is allowed to extendalong a mounting surface, which facilitates mounting.

As examples of the DC motors, there exist DC motors disclosed inJP-A-7-059322 (the term “JP-A” as used herein means an “unexaminedpublished Japanese patent application”) and JP-A-2000-279885,respectively.

In the example disclosed in JP-A-7-059322, field magnets are integrallymolded in an interior of an angularly cylindrical motor frame having asquare cross section. The field magnets are magnetized to exhibit fourequal magnetic poles which are polarized, respectively, to an N pole andan S pole in an alternating fashion so that centers of the magneticpoles coincide with corner portions of the motor frame. The fieldmagnets are formed into a shape which is angular on an outside and iscurved into an arc-shape on an inside thereof. The field magnets arearranged in such a manner as to be in contact with the adjacent fieldmagnets. A rotor is disposed inside the field magnets so arranged.

FIG. 7 is a drawing which illustrates a conventional configuration offield magnets disclosed in JP-A-2000-279885.

In FIG. 7 illustrated in JP-A-2000-279885, a rectangular prism-shapedmotor frame comprises an upper motor frame 101 and a lower motor frame(not illustrated) which are made of a metallic material. A plurality offield magnets 102, which are each externally angular and internallycircular, are provided on inner walls of the upper motor frame 101. Thefield magnets 102 are installed, respectively, at four corner portionsof the quadrangular upper motor frame 101 and comprise four fieldmagnets 102 a, 102 b, 102 c and 102 d which are each formed into a shapewhich is angular on an outside and is formed into an arc-shape on aninside thereof. The field magnets 102 are disposed adjacent to eachother via a minute gap Δg. A rotor (not illustrated) is disposed insidethe field magnets 102 so disposed.

In the example of JP-A-2000-279885, the field magnets 102 which are eachexternally angular and internally curved in an arc are disposed at thecorner portions of the upper motor frame. This configuration ispreferable in that necessary magnetization characteristics can beobtained. In this configuration, however, not only an air gap necessaryfor the armature core to rotate smoothly but also the field magnets 102are interposed between the motor frame and an armature core. Thus, sincethe field magnets 102 are so interposed, a length from an axial centerof the armature core to a radially outer side thereof is reduced by aradial thickness of the field magnet 102. As a result, the effectivemagnetic flux per slot of the armature core is lowered, leading to astate where torque generated in the armature core does not workeffectively. Furthermore, the number of times of winding a winding whichcan be wound around the armature core is reduced, and torque generatedin the armature core is lowered by such an extent.

Also, in the example of JP-A-7-059322, as in the example ofJP-A-2000-279885, the field magnets as well as a necessary air gap areinterposed between the motor frame and an armature core. Due to this, inthe example of JP-A-7-059322, as in the example of JP-A-2000-279885,torque generated in the armature core becomes small.

SUMMARY OF THE INVENTION

An objection of the invention is to provide a small DC motor configuredsuch that field magnets (hereinafter also simply referred to as“magnets”) are provided in a motor frame having the same number ofcorner portions as the number of polarized magnetic poles so as toincrease torque to be generated while keeping the shape of the small DCmotor (hereinafter also simply referred to as a “motor”) small withoutincreasing the same.

According to a small DC motor of the invention, an outside diameter ofan armature assembly having a rotating element such as an armature coreor an armature windings-molded element is made to be a value resultingwhen a necessary minimum air gap for the armature assembly to rotatesmoothly is subtracted from an inside diameter of a motor frame so as toincrease the outside diameter of the armature assembly to therebyincrease the effective magnetic flux of the armature assembly, so thattorque to be generated can be increased. In particular, when thearmature assembly has an armature core, an outside diameter of thearmature core is increased so as to increase the number of times ofwinding a winding around a pole formed between slots of the armaturecore to thereby increase the effective magnetic flux thereof, so thattoque to be generated can be increased.

In addition, when the armature assembly has an armature windings-moldedelement, an outside diameter of the armature windings-molded element isincreased so as to increase the number of times of winding an armaturewinding to thereby increase torque to be generated. In addition, anarmature assembly of any shape can be used as an armature assembly thatis to be incorporated in a motor frame having field magnets.

A reduction in the overall size of the motor can be accomplished withoutreducing the thickness of central portions of the poles of the fieldmagnets by forming the shape of the motor frame into a quadrangularshape having the same number of corner portions as the number ofpolarized magnetic poles of the field magnets. Furthermore, the shape ofthe motor frame can be formed into a shape with 2(n+1) sides and angles,where n is a positive integer of 1 or larger.

Furthermore, since the armature assembly such as the armature core orthe armature windings-molded element is allowed to be accommodatedwithin a necessary minimum space by making the outside diameter of thearmature assembly be the value resulting when one dimension of theminimum necessary air gap (as an actual dimension, an extremely shortarbitrary dimension of the order of 0.1 mm to 0.5 mm) from the insidediameter of the motor frame, the size of the motor can be reduced.

The radius of an arc on the inside of the corner portion of the motorframe is set to any value in the range of 5% to 85% of a length from acenter of the shaft to an arc-shaped surface on a shaft side of thefield magnet. Preferably, the radius of the arc on the inside of thecorner portion of the motor frame is set to any value in the range of65% to 85% of the length from the center of the shaft to the arc-shapedsurface on the shaft side of the field magnet.

By incorporating the field magnet configuration of the invention into acoreless motor or non-slotted cored motor, the size of the motor can bereduced and torque to be generated can be increased due to the aforesaidreason.

Specific means of the invention is described below.

-   (1) A small DC motor comprising: a motor frame comprising a    cylindrical portion, the cylindrical portion having a constant    thickness and having a cross section in a shape that comprises four    sides and connecting portions, each of the connecting portions    connecting adjacent two of the four sides and being located inward    from a corresponding corner in a quadrangle comprising the four    sides; field magnets having an arc-shaped surface on an inside    thereof and having a cross section in an inside portion that has an    arc-shape and in an outside portion that has conformable contact    with an inside surface of the motor frame; and an armature assembly    comprising: an armature winding and a shaft and being disposed    rotatably within the motor frame, wherein the field magnets are    provided so as to be spaced apart from each other, and the small DC    motor comprises an air gap between each of the four sides and a    radially outermost surface of the armature assembly, the air gap    being a necessary minimum size to rotate the armature assembly.-   (2) A small DC motor as described in item (1), wherein the armature    assembly comprises an armature core, the armature core constituting    the radially outermost surface of the armature assembly.-   (3) A small DC motor as described in item (1), wherein the armature    assembly comprises an armature windings-molded element, the armature    windings-molded element constituting the radially outermost surface    of the armature assembly.-   (4) A small DC motor as described in any one of the items (1) to    (3), wherein the field magnets comprise joining portions, each of    the joining portions joining the inside portion of the field magnets    to the outside portion of the field magnets and being disposed at    the air gap.-   (5) A small DC motor as described in any one of the items (1) to    (4), which comprises a shaft, wherein a radius from a center of the    shaft to the inside surface of the motor frame is the same as a    radius from the center of the shaft to the arc-shaped surface of the    field magnets.-   (6) A small DC motor as described in any one of the items (1) to    (4), wherein a radius from a center of a shaft to the inside surface    of the motor frame is the same as a shortest radius from the center    of the shaft to the arc-shaped surface of the field magnets, the    arc-shaped surface having an arbitrary curvature.-   (7) A small DC motor as described in any one of the items (1) to    (4), wherein each of the connecting portions have an arc-shape.-   (8) A small DC motor as described in any one of the items (1) to    (4), wherein each of the connecting portions have an arc-shape, the    arc-shape having an arbitrary ratio to a radius of the arc-shaped    surface of the field magnets.-   (9) A small DC motor as described in any one of the items (1) to    (4), wherein each of the connecting portions has a straight    line-shape.-   (10) A small DC motor as described in any one of the items (1) to    (4), wherein the cross section of the motor frame has an arc-shape,    the arc-shape constituting a portion of a circle, and a center of    the circle coincides with a center of the shaft.-   (11) A small DC motor as described in any one of the items (3) to    (10), wherein the armature assembly comprises: an armature    windings-molded element of a cylindrical coil wound by a magnet    wire; and an inner yoke comprising a cylindrical portion, the inner    yoke being fixed in such a manner to face the armature    windings-molded element.-   (12) A small DC motor as described in any one of the items (3) to    (10), wherein the armature assembly comprises: an armature    windings-molded element of a cylindrical coil wound by a magnet    wire; and a cylindrical movable back yoke having contact with an    inner side of the armature windings-molded element.

The invention provides the following advantages.

Since the invention provides a small DC motor including: a motor frameincluding a cylindrical portion, the cylindrical portion having aconstant thickness and having a cross section in a shape that includesfour sides and connecting portions, each of the connecting portionsconnecting adjacent two of the four sides and being located inward froma corresponding corner in a quadrangle including the four sides; fieldmagnets having an arc-shaped surface on an inside thereof and having across section in an inside portion that has an arc-shape and in anoutside portion that has conformable contact with an inside surface ofthe motor frame; and an armature assembly including: an armature windingand a shaft and being disposed rotatably within the motor frame, whereinthe field magnets are provided so as to be spaced apart from each other,and the small DC motor includes an air gap between each of the foursides and a radially outermost surface of the armature assembly, the airgap being a necessary minimum size to rotate the armature assembly, theoutside diameter of an outermost side of the armature assembly can bemade to be a value resulting when a necessary minimum air gap issubtracted from a shortest inside diameter of the motor frame. As aresult, a radial length of the armature assembly can be increased, andthis enables the extension of a winding space on the armature assembly,increase in torque to be generated, minimization of the quantity ofexpensive magnets to be used and reduction in size of the motor.

In the event that the armature assembly has the armature core, thelength of slots formed in the armature core can be increased, and thisenables the extension of a winding space between the slots, so as toincrease the number of times of winding a winding in the slots tothereby increase torque to be generated. In addition, due to this, thewinding space can be increased, and by maximizing the diameter of thearmature core, the torque can be increased, thereby making it possibleto minimize the quantity of expensive field magnets to be used and toreduce the size of the motor.

Since the air gap is determined by accuracies of two components, thatis, the accuracy of the inside diameter of the motor frame and theaccuracy of the outside diameter of the armature core, the air gap canbe configured in an extremely short arbitrary dimension of the order of0.1 mm to 0.5 mm as an actual dimension.

In the event that the armature assembly has the armature windings moldedelement, a radial length of the armature windings molded element can beincreased, and this enables an increase in winding space, increase intorque to be generated, minimization of the quantity of expensive fieldmagnets to be used, and reduction in size of the motor.

By forming the shape of the motor frame into a quadrangular shape havingthe same connecting portions (hereinafter also referred to as “cornerportions”) as the number of polarized magnetic poles of the fieldmagnets, the reduction in the overall size of the motor can beaccomplished without reducing the thickness of central portions of themagnetic poles of the field magnets. A basic shape of the motor frame isa quadrangular shape, and the shape of the cross section of the motorframe which results when corner portions of the quadrangular shape arecollapsed inwardly while part of each of sides thereof is left intact isformed into a shape in which the adjacent sides of the quadrangularshape which are spaced apart from each other are connected by arcs of anarbitrary shape. The basic shape is not limited to the quadrangularshape but can be formed into a shape with 2(n+1) sides and angles, wheren is a positive integer of 1 or larger. According to this configuration,the basic shape (the shape resulting before the corner portions arecollapsed) of the cylindrical portion of the motor frame can be formedinto, for example, a quadrangular shape, a hexagonal shape, an octagonalshape, . . . .

The radius of an arc on the inside of the corner portion of the motorframe is set to any value in the range of 5% to 85% of a length from thecenter of the shaft to an arc-shaped surface on a shaft side of thefield magnet. According to this configuration, the armature core can bedisposed with an enlarged construction without being restricted by thefield magnets with respect to a location where the armature core isplaced, and consequently, torque to be generated can be increased byincreasing the number of times of winding the armature winding.

Preferably, the radius of the arc on the inside of the corner portion ofthe motor frame is set to any value in the range of 65% to 85% of thelength from the center of the shaft to the arc-shaped surface on theshaft side of the field magnets.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention disclosed herein will be understood better with referenceto the following drawings of which:

FIGS. 1A to 1D are drawings which illustrate the configuration of asmall DC motor according to Embodiment 1 of the invention;

FIGS. 2A to 2D are drawings which illustrate the configuration of asmall DC motor according to Embodiment 2 of the invention;

FIGS. 3A to 3C are drawings which illustrate the configuration of asmall DC motor according to a further embodiment of the invention;

FIGS. 4A and 4B each are sectional views of a coreless motor to whichthe configuration of the field magnets of the invention is applied;

FIG. 5 is a sectional view of a small DC motor having another shape ofthe motor frame of the invention;

FIGS. 6A and 6B each are sectional views of a slotless cored motor towhich the configuration of the field magnets of the invention isapplied; and

FIG. 7 is drawing which illustrates a conventional configuration offield magnets disclosed in JP-A-2000-279885.

DETAILED DESCRIPTION OF THE INVENTION

Exemplary embodiments of the invention will be described in detail basedon the accompanying drawings.

Embodiment 1

FIGS. 1A to 1D are drawings which illustrate the configuration of asmall DC motor according to Embodiment 1 of the invention. In Embodiment1, a case will be described in which an armature assembly has anarmature core.

FIG. 1A is a sectional view taken along a longitudinal direction of ashaft, and in FIG. 1A, a segment from line A-A to line B-B is asectional view taken along line D-D in FIG. 1B, a segment from line B-Bto line C-C is a sectional view taken along line E-E in FIG. 1C, and asegment from line C-C to an end block is a sectional view taken alongline F-F in FIG. 1D. FIG. 1B is a sectional view taken along line A-A inFIG. 1A, FIG. 1C is a sectional view taken along line B-B in FIG. 1A,and FIG. 1D is a sectional view taken along line C-C in FIG. 1A.

A small DC motor 1 illustrated in FIG. 1A has an armature assembly 22and a frame assembly 23.

The armature assembly 22 has a shaft 2, an armature core 3 provided onthe shaft 2, armature windings 6 wound in slots 5 formed on both sidesof salient poles 4 on the armature core 3, and a commutator unit 9provided on the shaft 2.

A current-limiting varistor 7 is installed and a commutator piece 8 isprovided on the commutator unit 9 provided on the shaft 2. One end ofthe shaft 2 is rotatably supported in a bearing 12 mounted in a centralopening hole in an end plate portion 11 of a motor frame 10, and theother end thereof is rotatably supported in a bearing 14 mounted in anend block 13. Mounting threaded holes 15 are symmetrically provided inthe end plate portion 11 with respect to the shaft 2. A terminal 17having a brush 16 is provided in the end block 13. The brush 16 isdisposed at a position where it contacts the commutator piece 8.

The motor frame 10 is made up of a cylindrical portion 20 which has aconstant thickness and which has a cross section in a shape thatincludes four sides 18 and corner portions 19, each of the cornerportions 19 connecting adjacent two of the four sides 18 and beinglocated inward from a corresponding corner in a quadrangle including thefour sides 18, that is, the shape of the cross section is formed into ashape having a cross section resulting when corner portions 19 of aquadrangular shape are collapsed inwardly while part of each of sides 18of the quadrangular shape is left intact and the end plate portion 11which is provided continuously to the cylindrical portion 20. Whenviewed in cross section, the cylindrical portion 20 is such that theadjacent but spaced-apart sides 18 are connected by arc-shapes(hereinafter also referred to as “arcs”).

By forming the shape of the cylindrical portion 20 of the motor frame 10into basically a quadrangular shape having the same number of cornerportions as the number of polarized magnetic poles of field magnets 21,a configuration to obtain sinusoidal magnetization characteristics in arotating direction can be accomplished without reducing the thickness ofcentral portions of the magnetic poles of the field magnets, therebymaking it possible to reduce the size of the motor.

The outside diameter of the armature core 3 is set to the insidediameter of the cylindrical portion 20 of the motor frame 10 such thatonly a necessary minimum air gap G exists at a position where an outsidesurface of the armature core 3 comes nearest to an inside surface 10 aof the cylindrical portion 20 of the motor frame 10, that is, at aposition of a central portion of each side 18 in the event ofEmbodiment 1. In the example illustrated in FIG. 1, this setting iscarried out at the central portion of each side 18.

The air gap G is determined by accuracies of two components, that is,the accuracy of an inside diameter of the motor frame 10 and theaccuracy of an outside diameter of the armature core 3. Due to this, theair gap G takes a value of the order of 0.1 mm to 0.5 mm as an actualdimension.

The radius of an arc on an inside of the corner portion 19 of the motorframe 10 is set to any value from 5% to 85% of a length from a center ofthe shaft 2 to an arc-shaped surface of an inside portion 21 a of thefield magnet 21 which lies on a shaft 2 side thereof. According to thisconfiguration, the armature core 3 can be disposed as being enlarged inconstruction without being restricted by the field magnets 21 withrespect to a location where the armature core 3 is disposed, andconsequently, the number of times of winding the armature winding 6 canbe increased so as to increase torque to be generated. Preferably, theradius of the arc on the inside of the corner portion 19 of the motorframe 10 is set to any value from 65% to 85% of the length from thecenter of the shaft 2 to the arc-shaped surface of the inside portion 21a of the field magnet 21 which lies on the shaft 2 side thereof.

The embodiment includes the following preferred features.

The radius from the center of the shaft 2 to the inside surface 10 a ofthe motor frame 10 is the same as a radius from the center of the shaft2 to the arc-shaped surface of the inside portion 21 a of the fieldmagnets 21.

In addition, the radius from the center of the shaft 2 to the insidesurface 10 a of the motor frame 10 is the same as a shortest radius fromthe center of the shaft 2 to an arc-shaped surface of the inside portion21 a of the field magnets 21 which has an arbitrary curvature.

Additionally, each of the connecting portions 19 has an arc-shape, thatis, the cross section of the motor frame 10 which results when thecorner portions 19 of the motor frame 10 are collapsed inwardly whilepart of each of the sides 18 of the quadrangular shape is left intact isformed into a shape in which the adjacent sides 18 of the quadrangularshape which are spaced apart from each other are connected by arcs of anarbitrary shape.

In addition, each of the connecting portions 19 has an arc-shape, thearc-shape having an arbitrary ratio of a radius of the arc-shapedsurface of the field magnets 21, that is, the cross section of the motorframe 10 which results when the corner portions 19 of the motor frame 10are collapsed inwardly while part of each of the sides 18 of thequadrangular shape is left intact is formed into a shape in which theadjacent sides 18 of the quadrangular shape which are spaced apart fromeach other are connected by arcs of any ratio to the length of a radiusof an arc (having a radius from the center of the shaft 2 to thearc-shaped surface of the field magnet) which forms the arc-shapedsurface.

Additionally, each of the connecting portions 19 has a straightline-shape, that is, the cross section of the motor frame 10 whichresults when the corner portions 19 of the motor frame 10 are collapsedinwardly while part of each of the sides 18 of the quadrangular shape isleft intact is formed into a shape in which the adjacent sides 18 of thequadrangular shape which are spaced apart from each other are connectedby straight lines.

In addition, the cross section of the motor frame 10 which results whenthe corner portions 19 of the motor frame 10 are collapsed inwardlywhile part of each of the sides 18 of the quadrangular shape is leftintact is formed into a shape in which the adjacent sides 18 of thequadrangular shape which are spaced apart from each other are connectedby arcs of a concentric circle with respect to the center of the shaft2. The end block 13 is mounted on an open end of the cylindrical portion20 which lies opposite to the end plate portion 11 of the motor frame10.

The frame assembly 23 is made up of the motor frame 10, the bearing 12and the field magnets 21.

The field magnet 21 is made of, for example, a neodymium magnet(Nd—Fe—B) or the like and is magnetized in a radial direction orrotating direction, and the field magnets are disposed in fourrespectively at the corner portions 19 of the cylindrical portion 20having the quadrangular cross section in such a manner as to be spacedapart from each other.

A cross section of the field magnet 21 (a cross section of a plane whichintersects the longitudinal direction of the shaft 2 at right angles) isformed into a shape in which the inside portion (the side which liescloser to the armature core 3) 21 a is formed into an arc-shape, whilean outside portion 21 b is made to firmly secure to the inside surface10 a of the cylindrical portion 20 of the motor frame 10. A joiningportion 21 c between the inside portion 21 a and the outside portion 21b is formed at an angle at which the outside portion 21 b intersects theinside surface 10 a of the motor frame 10 in Embodiment 1, but thejoining portion 21 c can also be formed at any angle to the insidesurface 10 a of the motor frame 10. The field magnet 21 has within theair gap G defined between the inside surface 10 a of the motor frame 10and a radially outermost surface of the armature core 3 such joiningportions 19 which connect, respectively, the inside portion 21 a of thefield magnet 21 which is formed into the arc-shape as viewed in crosssection and the outside portion 21 b thereof which firmly secures to theinside surface 10 a of the motor frame 10 as viewed in cross section.The field magnet 21 is, as illustrated in FIG. 1A, disposed in thelongitudinal direction of the cylindrical portion 20 in such a manner asto face the armature core 3.

In particular, in the small DC motor 1, in order to optimize arelationship between a maximum energy product (BH) max of the fieldmagnet 21 which largely affects the motor properties and the outsidediameter dimension of the armature core 3 which makes up a magnetismforming portion of the rotating element, by increasing the outsidediameter of the armature core 3 to a value resulting when the necessaryminimum air gap G is subtracted from the inside diameter of thecylindrical portion 20 of the motor frame 10, the effective magneticflux per slot of the armature core 3 can be increased, and furthermore,a winding region in the slots 5 of the armature core 3 can be increased,and the quantity of field magnets 21 to be used, which are expensive asa motor component, can be suppressed, whereby torque to be generated isincreased while realizing the reduction in dimensions of the motor, sothat the reduction in the cost and volume of the motor is realized.

In the event that a magnet having a large maximum energy product isused, the optimization of dimensions of the magnet is required inconsideration of countermeasures against cogging torque, and theinvention exhibits tremendous effect on the solution of a problemrelating to the cogging torque.

Embodiment 2

FIGS. 2A to 2D are drawings which illustrate the configuration of asmall DC motor according to another embodiment of the invention.

FIG. 2A is a sectional view taken along a longitudinal direction of ashaft, and in FIG. 2A, a segment from line H-H to line I-I is asectional view taken along line K-K in FIG. 2B, a segment from line I-Ito line J-J is a sectional view taken along line L-L in FIG. 2C, and asegment from line J-J to an end block is a sectional view taken alongline M-M in FIG. 2D.

FIG. 2B is a sectional view taken along line H-H in FIG. 2A, FIG. 2C isa sectional view taken along line I-I in FIG. 2A, and FIG. 2D is asectional view taken along line J-J in FIG. 2A.

FIG. 2 has the same constituent elements except for field magnets asthose of FIG. 1. Like reference numerals are given to like constituentelements, and the description thereof will be omitted.

A small DC motor 1 illustrated in FIG. 2A has an armature assembly 22and a frame assembly 23A.

The frame assembly 23A is made up of a motor frame 10, a bearing 12 andfield magnets 21A which are connected together by a connecting frameportion 21 d.

As illustrated in FIG. 2D, the field magnet 21A is formed so as to have,as viewed in cross section, an inside portion 21 a on which anarc-shaped surface is formed with a radius from a center of a shaft 2and an outside portion 21 b which is formed so as to firmly secure to aninside surface 10 a of the motor frame 10. The field magnet 21A isconfigured so as to have joining portions 21 c which each connect, asviewed in cross section, the inside portion 21 a and the outside portion21 b thereof within an air gap G defined between the inside surface 10 aof the motor frame 10 and an outside surface of an armature core 3. Thefield magnets 21A are disposed in four in a cylindrical portion 20 ofthe motor frame 10 which has across section in a shape that comprisesfour sides and connecting portions, each of the connecting portionsconnecting adjacent two of the four sides and being located inward froma corresponding corner in a quadrangle comprising the four sides, thatis, a cross section resulting when corner portions 19 of a quadrangularshape are collapsed inwardly while part of each of sides 18 of thequadrangular shape is left intact, the four field magnets 21A beingdisposed respectively in the resulting corner portions 19 in such amanner as to be spaced apart from each other.

In particular, in the small DC motor 1, in a relationship between amaximum energy product (BH)max of the field magnet 21A which largelyaffects the motor properties and an outside diameter of the armaturecore 3 which makes up a magnetism forming portion of the rotatingelement, by increasing the outside diameter of the armature core 3 to avalue resulting when a necessary minimum air gap G is subtracted from aninside diameter of the motor frame 10, the effective magnetic flux perslot of the armature core 3 can be increased, and furthermore, a windingregion in the slots 5 of the armature core 3 can be increased, and thequantity of field magnets 21A to be used, which are expensive as a motorcomponent, can be suppressed, whereby torque to be generated isincreased while realizing the reduction in dimensions of the motor, andthe number of man-hours for assembling the motor can be reduced byintegrating the field magnet 21A into one piece, so that the reductionin the cost and volume of the motor is realized.

As illustrated in FIGS. 2A and 2C, the connecting frame portion 21 d isconfigured substantially into a quadrangular shape along the insidesurface 10 a of the cylindrical portion 20 of the motor frame 10. Theconnecting frame portion 21 d is provided in such a manner as to be incontact with an end plate portion 11 and the cylindrical portion 20.

Embodiment 3

FIGS. 3A and 3B are drawings which illustrate the configuration of asmall DC motor 1 according to a further embodiment of the invention. Inaddition, armature windings are omitted from the drawing. FIG. 3A is asectional view which illustrates a cross section of a motor frame havinga plurality of curved surfaces for 4 poles and 6 slots, FIG. 3B is asectional view which illustrates across section of an octagonal motorframe for 8 poles and 12 slots, and FIG. 3C is a sectional view whichillustrates a cross section of a square motor frame for 4 poles and 6slots.

(Shapes of Motor Frame)

The motor frames of the aforesaid embodiments take the quadrangularshape as the basic shape thereof and are formed into the shape in whichin the cross section of the motor frame resulting when the cornerportions are collapsed inwardly while part of each of the sides of thequadrangular shape is left intact, the adjacent but spaced-apart sidesof the motor frame are connected by the arcs of an arbitrary shape orthe like, the basic shape is not limited to the quadrangular shape butcan be formed into a shape with 2(n+1) sides and angles, where n is apositive integer of 1 or larger. According to this configuration, thebasic shape (the shape resulting before the corner portions arecollapsed) of the cylindrical shape can be formed into, for example, aquadrangular shape, a hexagonal shape, an octagonal shape, . . . .

In the small DC motor 1 of the invention, the shape of the cross sectionof the motor frame which results when corner portions 19 a of thequadrangular shape are collapsed inwardly while part of each of sides 18thereof is left intact may be formed into a shape in which the adjacentsides 18 of the quadrangular shape which are spaced apart from eachother are connected by arcs of an arbitrary shape, the shape of thecross section of the motor frame which results when the corner portions19 a of the quadrangular shape are collapsed inwardly while part of eachof the sides 18 thereof is left intact may be formed into a shape inwhich the adjacent sides of the quadrangular shape which are spacedapart from each other are connected by arcs of arbitrary ratio to thelength of a radius of an arc which forms the arc-shaped surface, theshape of the cross section of the motor frame which results when thecorner portions 19 of the quadrangular shape are collapsed inwardlywhile part of each of the sides 18 thereof is left intact may be formedinto a shape in which the adjacent sides of the quadrangular shape whichare spaced apart from each other are connected by straight lines, or theshape of the cross section of the motor frame which results when thecorner portions 19 of the quadrangular shape are collapsed inwardlywhile part of each of the sides 18 thereof is left intact may be formedinto a shape with 2(n+1) sides and angles, where n is a positive integerof 1 or larger. The radius of an arc on an inside of a field magnet 21 fas viewed in cross section is made to continue to be extended so thatthe arc is made to be becoming almost a straight line, for example,whereby the magnetization characteristics in the rotating direction ofthe field magnet 21 f can be formed into sinusoidal shape.

Also in examples illustrated in FIGS. 3A to 3C, the field magnet 21 fhas joining portions 21 c which each connect, as viewed in crosssection, an inside portion 21 a of the field magnet 21 f which is formedinto an arc-shape and an outside portion 21 b thereof which is providedso as to firmly secure to an inside surface 10 a of a motor frame 10within an air gap G defined between the inside surface of the motorframe 10 and a radially outermost surface of an armature core 3.

In addition, the shape of the armature core can be alteredappropriately. While in this embodiment, the armature core is configuredas having 4 poles and 6 slots, the armature core can be configured ashaving 4 poles and 5 slots.

In the example illustrated in FIG. 3A, as illustrated in a cross sectiontherein, a motor frame 10 d is configured based on a circular shape onthe whole by increasing the radius of arcs of the corner portions 19 aof the motor frame 10 while part of each side 18 thereof is left intact.Field magnets 21 f are each configured such that an inside portion 21 ais formed into an arc-shaped surface while an outside portion 21 b ismade to firmly secure to an inside surface 10 a of the motor frame 10 dand are provided respectively at the corner portions 19 a of the motorframe 10 d in such a manner as to be spaced apart from each other sothat the field magnets 21 f do not obstruct the rotating operation ofthe armature core 3, whereby the field magnets 21 f are each formed intoa shape which can secure magnetization characteristics of each magnetnecessary to maintain the motor properties.

In the example illustrated in FIG. 3B, as illustrated in a cross sectiontherein, there is provided a motor shape for 8 poles and 12 slots, andthe cross section of a motor frame 10 e which results when cornerportions 19 b of a quadrangular shape are collapsed inwardly while partof each side 18 thereof is left intact is formed into an irregularoctagon with the straight sides 18 kept remaining. Field magnets 21 gare each configured such that an inside is formed into an arc-shapedsurface while an outside is made to firmly secure to an angle formed bytwo straight lines of the motor frame 10 e and are provided respectivelyat the corner portions 19 b of the motor frame 10 e in such a manner asto be spaced apart from each other so that the field magnets 21 g do notobstruct the rotating operation of the armature core 3, whereby thefield magnets 21 g are each formed into a shape which can securemagnetization characteristics of each magnet necessary to maintain themotor properties.

In the example illustrated in FIG. 3C, as illustrated in a cross sectiontherein, the radius (the corner R) of corner portions (corners) 19 c offour sides 18 of a motor frame 10 f is set such that a ratio of theradius of the corner R to an inner radius of an arc-shaped surface of afield magnet 21 h (a radial length from the center of a shaft 2 to thearc-shaped surface of the field magnet) falls within a range from 5% to85%. Preferably, the radius of the arc of the individual corner portionsof the motor frame is set to any value in the range of 65% to 85% of thelength from the center of the shaft 2 to the arc-shaped surface of thefield magnet which lies on a shaft side thereof.

The field magnets 21 h are each configured such that the inside portion21 a is formed into an arc-shaped surface while the outside portion 21 bis made to firmly secure to the motor frame 10 f and are providedrespectively at the corner portions 19 c of the motor frame 10 f in sucha manner as to be spaced apart from each other so that the field magnets21 h do not obstruct the rotating operation of the armature core 3,whereby the field magnets 21 h are each formed into a shape which cansecure magnetization characteristics of each magnet necessary tomaintain the motor properties.

Embodiment 4

FIGS. 4A and 4B are sectional views which illustrate a coreless motor towhich the magnet configuration of the invention is applied.

FIG. 4A is a sectional view taken along line P-P in FIG. 4B, and FIG. 4Bis a sectional view taken along line N-N in FIG. 4A. A coreless motor 29of the invention is involved in the scope of the small DC motor of theinvention.

The coreless motor 29 includes an armature assembly rotatably disposedwithin a motor frame 30, the motor frame 30 including a commutator mold37, an armature windings-molded element 38 and a shaft 2.

The motor frame 30 of the coreless motor 29 is made up of a cylindricalportion 31, an end plate portion 32 which is provided continuously tothe cylindrical portion 31, and a bearing support portion 33 which isprovided continuously to the endplate portion 32. In an interior of thebearing support portion 33, the shaft 2 is rotatably supported by twobearings 34, 35 which are separated from each other so as to be providedat upper and lower ends of the bearing support portion 33, respectively.One end of the shaft 2 is brought into abutment with a bottom plate 36.The commutator mold 37 is provided on the shaft 2.

The commutator mold 37 is made up of a cylindrical portion 37 a which ismade of resin and is provided in such a manner as to surround thecircumference of the bearing support portion 33 of the motor frame 30,an inner annular plate portion 37 b which is provided continuously to anend of the cylindrical portion 37 a and is fixed to the shaft 2 and anouter annular plate portion 37 c which is provided on the other end ofthe cylindrical portion 37 a in such a manner as to protrude radiallyoutwardly therefrom, and a commutator piece 39 a and a riser 39 b whichcontinues to the commutator piece 39 a to constitute a feeding path tothe armature winding-molded element 38 are partially molded in thecommutator mold 37 in such a manner as to be embedded integrallytherewith. A radially outside of the riser 39 b electrically connects toand supports with a required strength the armature winding-moldedelement 38 via a tap 38 a made of a metallic elongated piece. A pair ofbrushes 40, which are to be brought into contact the commutator piece 39a, are mounted on a terminal 41 fixed to the bottom plate 36.

The armature winding-molded element 38 takes a configuration in which acoil into which a magnet wire is wound is formed into a thin cylindricalshape and is then fixed with a resin, taking in the tap 38 a on its wayto the top so as to connect to the riser 39 b via the tap 38 a. An inneryoke 42 and field magnets 43 are disposed on a stationary side in such amanner as to hold the armature winding-molded element 38 on a rotatingside therebetween. The inner yoke 42 is made up of a cylindrical portion42 a which faces the armature winding-molded element 38 and a bentportion 42 b which is disposed on the bottom plate 36. The field magnets43 are disposed respectively at corner portions 45 which connecttogether individual sides 44 of the motor frame 30 in such a manner asto be spaced apart from each other.

The cylindrical portion 42 a of the inner yoke 42 short-circuitsmagnetic flux generated in the field magnets 43 to thereby lower leakageflux.

The motor frame 30 is made up of the cylindrical portion 31 which has aconstant thickness and which is formed into a shape having acrosssection resulting when corner portions 45 of a quadrangular shape arecollapsed inwardly while part of each of the sides 44 of thequadrangular shape is left intact, the end plate portion 32 which isprovided continuously to the cylindrical portion 31 and the bearingsupport portion 33 which is provided continuously to the end plateportion 32. In the cross section of the cylindrical portion 31, thesides 44 are connected to each other by arcs at the corner portions 45.The bearing support portion 33 is formed into a cylindrical shape.

The reduction in size of the motor can be achieved without reducing thethickness of central portions of poles of the field magnets 43 byforming the shape of the cylindrical portion 31 of the motor frame 30basically into a quadrangular shape which has the same number of cornerportions as the number of polarized magnetic poles of the field magnets43.

The outside diameter of the armature winding-molded element 38 is set tothe inside diameter of the cylindrical portion 31 of the motor frame 30such that only a necessary minimum air gap G is provided at a positionwhere a radially outermost surface of the armature winding-moldedelement 38 comes nearest to a radially innermost surface of thecylindrical portion 31, that is, at a position of a central position ofeach side in the event of this embodiment. In the example illustrated inFIG. 4, this setting is carried out at the central portion of each side44.

The air gap G is determined mainly by accuracies of two constituentcomponents, that is, the accuracy of the inside diameter of the motorframe 30 and the accuracy of the outside diameter of the armaturewinding-molded element 38. Due to this, the air gap G takes a value ofthe order of 0.1 mm to 0.5 mm as an actual dimension.

The radius of the arc on an inside of the corner portion 45 is set toany value in the range of 5% to 85% of a length from the center of theshaft 2 to an arc-shaped surface of an inside portion 43 a of the fieldmagnet 43 which lies on a shaft 2 side thereof, whereby the armaturewinding-molded element 38 can be disposed as being enlarged in diameterwithout being restricted by the field magnets 43 with respect to alocation where it is disposed, and consequently, the number of times ofwinding an armature winding can be increased so as to increase torque tobe generated. Preferably, the radius of the arc on the inside of thecorner portion 45 of the motor frame 30 is set to any value in the rangeof 65% to 85% of the length from the center of the shaft 2 to thearc-shaped surface of the inside portion 43 a of the field magnet 43which lies on the shaft 2 side thereof.

The field magnets 43 are made of, for example, neodymium magnets(Nd—Fe—B) or the like, are magnetized in a radial direction or rotatingdirection, and are disposed at the corner portions 45 of the cylindricalportion 31 having the quadrangular cross section in such a manner as tobe spaced apart from each other.

The cross section of the field magnet 43 is formed into a shape in whichan inside portion 43 a (a side which lies close to the armaturewinding-molded element 38) exhibits an arc-shape formed with a radiusfrom the center of the shaft 2, while an outside portion 43 b is made tofirmly secure to an inside surface 31 a of the cylindrical portion 31 ofthe motor frame 30. A joining portion 43 c between the inside portion 43a and the outside portion 43 b is formed at an angle at which itintersects the inside surface 31 a of the cylindrical portion 31 of themotor frame 30 at right angles, but the joining portion 43 c can also beformed at an arbitrary angle.

The field magnet 43 has the joining portions 43 c which each connecttogether, as viewed in cross section, the inside portion 43 a of thefield magnet 43 which is formed into the arc-shape and the outsideportion 43 b thereof which is made to firmly secure to the insidesurface 31 a of the motor frame within the air gap G defined between theinside surface 31 a of the motor frame 30 and the radially outermostsurface of the armature winding-molded element 38.

In particular, in a small DC motor 1, in order to optimize arelationship between a maximum energy product (BH) max of the fieldmagnet 43 which largely affects the motor properties and the outsidediameter dimension of the armature winding-molded element 38 which makesup a magnetism forming portion of the rotating element, by increasingthe outside diameter of the armature winding-molded element 38 to avalue resulting when the necessary minimum air gap G is subtracted fromthe inside diameter of the cylindrical portion 31 of the motor frame 30,the effective magnetic flux of the armature winding-molded element 38can be increased, and furthermore, a winding region of the armaturewinding-molded element 38 can be increased, and the quantity of fieldmagnets 43 to be used, which are expensive as a motor component, can besuppressed, whereby torque to be generated is increased while realizingthe reduction in dimensions of the motor, so that the reduction in thecost and volume of the motor is realized.

The configuration of the motor frame 30 and the field magnets 43 can bealtered variously as has been described above.

In the coreless motor 29 of Embodiment 4, since the outside diameter ofthe armature winding-molded element 38 is set to the inside diameter ofthe cylindrical portion 31 of the motor frame 30 such that only thenecessary minimum air gap G is provided at the position where theradially outermost surface of the armature winding-molded element 38comes nearest to the radially innermost surface of the cylindricalportion 31, that is, at the position of the central position of eachside in the event of Embodiment 4, the radial length of the armaturewinding-molded element 38 can be increased, and due to this, the windingspace can be increased, so that the diameter of the armaturewinding-molded element 38 can be maximized to increase torque to begenerated, while minimizing the quantity of expensive field magnets 43to be used, whereby the size of the motor can be reduced.

In addition, since the shape of the cylindrical portion 31 of the motorframe 30 is formed into the quadrangular shape which has the same numberof corner portions as the number of polarized magnetic poles of thefield magnets 43, the size of the motor can be reduced without reducingthe thickness of the central portions of the poles of the field magnets43 which accomplishes the sinusoidal magnetization characteristics.

Additionally, since the shape of the commutator mold 37 is made up ofthe cylindrical portion 37 a which is provided in such a manner as tosurround the circumference of the bearing support portion 33 of themotor frame 30, the inner annular plate portion 37 b which is providedcontinuously to the end of the cylindrical portion 37 a and is fixed tothe shaft 2 and the outer annular plate portion 37 c which is providedon the other end of the cylindrical portion 37 a in such a manner as toprotrude radially outwardly therefrom, the two bearings 34, 35 can beprovided in such a manner as to be spaced apart from each other forstable support while securing a contact space between the commutatorpiece 39 a and the brushes 40, so as to provided an advantage where thecylindrical portion 42 a of the inner yoke 42 can be formed long in theaxial direction.

A resin reinforcement film can be provided on outside surfaces of theouter annular plate portion 37 c and the armature winding-molded element38 so as to make up a vibration-proof construction. The tap 38 a and theriser 39 b are connected together by welding. The tap 38 a is formedinto a strip-like shape and a curved portion is provided on part thereofin order to impart elasticity.

Since the field magnets 43 and the inner yoke 42 are provided to faceeach other while holding the armature winding-molded element 38therebetween, a magnetic path (mainly the inner yoke) of a magneticmaterial can be provided long, and the magnetic resistance of themagnetic path can be suppressed to a lower level, thereby making itpossible to suppress the reduction in flux density. In addition,needless to say, cogging torque can also be suppressed.

Embodiment 5

FIG. 5 is a cross section of a small DC motor having a further motorframe shape.

While the basic shapes of the motor frames which have been described inthe aforesaid embodiments are the quadrangular shape, and the shape ofthe cross section of the motor frame 10 which results when the cornerportions 19 of the quadrangular shape are collapsed inwardly while partof each of the sides 18 thereof is left intact is formed into the shapein which the adjacent sides 18 of the quadrangular shape which arespaced apart from each other are connected by the arc of any shape, thebasic shape is not limited to the quadrangular shape but can be formedinto a shape with 2(n+1) sides and angles, where n is a positive integerof 1 or larger. According to this configuration, the basic shape (theshape resulting before the corner portions are collapsed) of thecylindrical portion of the motor frame can be formed into, for example,a quadrangular shape, a hexagonal shape, an octagonal shape, . . . .

An example depicted in FIG. 5 illustrates a hexagonal motor frame 50which result when n is 2. The motor frame is formed in 6 poles and 9slots. A cylindrical portion 51 of the hexagonal motor frame 50 isformed such that a cross section of the motor frame 50 which resultswhen corner portions 52 of the hexagonal shape are collapsed inwardlywhile part of each of sides 53 thereof is left intact is formed into ashape in which any pair of the adjacent sides 53 of the motor frame 50which are spaced apart from each other are connected by an arc having anarbitrary ratio to the length of the radius of an arc which forms anarc-shaped surface on an inside portion 21 a of a field magnet 54. Inaddition, the cross section may be formed into a shape in which theadjacent sides 53 are so connected by a straight line.

The radius (the corner R) of the corner portion 52 of each side 53 ofthe motor frame 50 is set such that a ratio of the radius of the cornerR to an inner radius of an arc-shaped surface of the field magnet 54 (aradial length from the center of a shaft 2 to the arc-shaped surface ofthe field magnet) falls within the range of 5% to 85%. Preferably, theradius of the arc of the individual corner portions of the motor frameis set to any value in the range of 65% to 85% of the length from thecenter of the shaft 2 to the arc-shaped surface of the field magnetwhich lies on a shaft side thereof.

The field magnets 54 are each configured such that the inside portion 21a is formed into an arc-shaped surface while an outside portion 21 b ismade to firmly secure to the motor frame 50 and are providedrespectively at the corner portions 52 of the motor frame 50 in such amanner as to be spaced apart from each other so that the field magnets54 do not obstruct the rotating operation of an armature core 55,whereby the field magnets 54 are each formed into a shape which cansecure magnetization characteristics of each magnet necessary tomaintain the motor properties.

The field magnet 54 has joining portions 21 c which each connecttogether, as viewed in cross section, the inside portion 21 a of thefield magnet 54 which is formed into the arc-shape and the outsideportion 21 b thereof which is made to firmly secure to the motor frame50 within an air gap G defined between the inside surface of the motorframe 50 and a radially outermost surface of the armature core 55. Inaddition, the shape of the armature core 55 can be alteredappropriately.

Embodiment 6

FIGS. 6A and 6B illustrate sectional views of a slotless cored motor towhich the magnet arranging configuration of the invention is applied.

FIG. 6A is a sectional view taken along line R-R in FIG. 6B, and FIG. 6Bis a sectional view taken along line Q-Q in FIG. 6A. The slotless coredmotor of this embodiment is also involved in the scope of the small DCmotor of the invention.

A slotless cored motor 47 of the invention is configured such that, inplace of the inner yoke 42 of the coreless motor described in Embodiment4, a movable back yoke 46 is provided in such a manner as to be incontact with an inner side of an armature winding-molded element 38 andthe movable back yoke 46 is supported by a commutator mold 37. Themovable back yoke 46 is made of a resin into which a magnetic materialis mixed or a magnetic material, and the movable back yoke 46 has noslots and is formed into a cylindrical shape like the armaturewinding-molded element 38.

To indicate a magnetic flux path, for example, a magnetic flux from afield magnet 43MNL having an N pole passes through by way of thearmature winding-molded element 38, the movable back yoke 46, thearmature winding-molded element 38, a field magnet 43MSL having an Spole and a motor frame 30. Since a location where the magneticresistance becomes high is situated only at gaps between the magnet NMLand the magnet MSL and the armature winding-molded element 38 along thispath, a motor can be configured which has large flux density and largetorque.

In addition, since the movable back yoke 46 is provided inside thearmature winding-molded element 38, compared to the coreless motor 29 inEmbodiment 4, inertial force becomes large and smooth rotation isprovided.

While the embodiments have been described as being applied to the innerrotor motors, the invention can also be configured as an outer rotormotor in the event that a shaft is provided on a motor frame in whichfield magnets are provided, windings are fixed thereto and drivingcurrent is made to flow through the windings. In addition, the inventioncan also be configured as a generator in the event that an externalpower is imparted to the shaft for rotation.

The configurations that have been described heretofore can be combineddifferently in an appropriate fashion without altering the functions.

The armature assembly 22 is allowed to rotate both inside and outsidethe magnets which are arranged at a predetermined diverging angle.

In addition to the embodiments that have been described heretofore, theindividual configurations can be combined to configure small DC motorsof arbitrary properties.

The present application claims foreign priority based on Japanese PatentApplication (JP 2005-202300) filed Jul. 11 of 2005, Japanese PatentApplication (JP 2005-348028) filed Dec. 1 of 2005, the contents of whichis incorporated herein by reference.

1. A small DC motor comprising: a motor frame comprising a cylindricalportion, the cylindrical portion having a constant thickness and havinga cross section in a shape that comprises four sides and connectingportions, each of the connecting portions connecting adjacent two of thefour sides; field magnets, each being positioned adjacent to each of theconnecting portions, having an arc-shaped surface on an inside thereofand having a cross section in an inside portion that has an arc-shapeand in an outside portion that has a same curvature as an inside surfaceof the connecting portions; and an armature assembly comprising anarmature winding and a shaft and being disposed rotatably within themotor frame, wherein the field magnets are provided so as to be spacedapart from each other, wherein the small DC motor comprises an air gapbetween each of the four sides and a radially outermost surface of thearmature assembly, and wherein the each of the connecting portions ofthe motor frame has a arc-shape, a radius of the arc-shaped surface onthe inside of the connecting portions of the motor frame is set to anyvalue from 5% to 85% of a radius from a center of the shaft to thearc-shaped surface of the inside of the field magnet.
 2. A small DCmotor according to claim 1, wherein the radius of the arc-shaped surfaceon the inside of the connecting portions of the motor frame is set toany value from 65%, to 85% of the radius from the center of the shaft tothe arc-shaped surface of the inside of the field magnet.
 3. A small DCmotor according to claim 1, wherein a radius from a center of the shaftto an inside surface of the motor frame is substantially same as theradius from the center of the shaft to the arc-shape surface of theinside of the field magnets.
 4. A small DC motor according to claim 1,wherein field magnet has a maximum thickness portion at portionscorresponding to the connecting portions of the motor frame and aminimum thickness portion gradually decreasing from the maximumthickness portion toward circumferentially opposite end.
 5. A small DCmotor comprising: (a) a motor frame comprising a cylindrical portion,the cylindrical portion having a constant thickness and having a crosssection in shape that comprises 2 (n+1) side portions and cornerportions, each of the corner portions being located between the adjacenttwo of the 2 (n+1) side portions and connecting together with theadjacent two side portions, wherein n is a positive integer of 2 orlarger; (b) field magnets attached to an inner surface of the motorframe, each of the field magnets having an arc-shaped surface on aninside thereof and an outer surface conforming to an inner surface ofthe corner portions of the motor frame; and (c) an armature assemblycomprising an armature winding and a shaft and being disposed rotatablywithin the motor frame, (d) wherein the field magnets are provided so asto be spaced apart from each other, and the each of the field magnetshas a maximum thickness at portions corresponding to the corner portionsof the motor frame and a minimum thickness gradually decreasing from themaximum thickness portion toward circumferentially opposite end, (e)wherein the small DC motor comprises an air gap between each of thesides and a radially outermost surface of the armature assembly, the airgap being a minimum size needed to rotate the armature assembly, and (f)wherein a radius from the center of the shaft to an inside surface ofthe sides of the motor frame is substantially equal to a radius from thecenter of the shaft to the arc-shaped surface of the inside portion ofthe field magnets.
 6. A small DC motor according to claim 5, wherein thecylindrical portion of the motor frame is formed into a hexagonal shapewhen n is
 2. 7. A small DC motor according to claim 5, wherein thecylindrical portion of the motor frame is formed into an octagonal shapewhen n is 3.